Display panel having reduced tensile stress

ABSTRACT

The invention relates to a method for press-forming a cathode ray tube panel, in which, before the panel is removed from the press, a heat radiation controlling means is provided at a small distance from the panel, parallel to the panel surface. Said means comprises an area with a high reflectivity for heat radiation, which first area is near the center of the panel. The result of this measure during the cooling of a panel is a more even temperature distribution over the surface of the panel. In particular, the temperature gradient between center and edges of the panel is reduced. Because of the glass wedge of (Real) Flat panels the center of the panel is thinner than the edge of the panel. The thinner part will normally cool down faster because of the lower heat capacity. Local temperature differences over the surface of the panel during cooling down will lead to so-called ‘membrane’ stresses. The method of the invention will minimize these stresses.

The invention relates to a method for manufacturing a glass panel for adisplay tube comprising a first stage of press-forming molten glass putin a mold using a plunger and a second stage of cooling the formed glassafter it has been taken out from the mold.

In the known methods a glass panel is press-formed which usually takesplace at very high temperatures (1000° C.-1100° C.). In this manner aglass face panel can be formed. Cathode ray tubes, for example, comprisesuch a glass panel.

Cathode ray tubes (CRT's) are becoming of ever more greater size, thusincreasing the weight of the CRT's. Furthermore the front surface of theglass panel is becoming ever more flatter. However, increasing theflatness of the front surface of the face panel generally increases alsothe weight of the glass panel because the thickness of the glass panelhas to be increased to ensure safety against implosion or explosion ofthe CRT.

There exists a great need for increase in strength of the CRT, and inparticular of the glass panel, without increasing the weight. Anincrease in the strength of the glass panel may improve the yield.

The present invention is aimed at providing a method which enablesincreasing the yield of the method and/or reducing the weight of theglass panel.

To this end the method in accordance with the invention is characterizedin that during a part of the first stage after pull back of the plungerthe heat radiation of the inner face portions of the central panelportion is reduced to reduce a temperature gradient to the edge portionsof the panel.

The invention is based on the insight that during the cooling phaseafter forming in the mold inhomogeneities in the stress level in thepanel can occur. In order to improve the strength of (Real Flat) displaypanels the cooling velocity after press forming is usually increased toa such extent that a compressive stress is produced in the surface inthe order of 3 to 25 Mpa. Trying to correct for the aboveinhomogeneities in the stress level during a final annealing stage wouldeasily affect the necessary compressive stress (bulk stress). Thisreduces the yield and can seriously affect the safety of the tubes. Thisis in particular important for panels with an (almost) flat inner and/orouter surface such as Real Flat panels.

Because real flat panels (for instance 51 RF) have a wedge, normally theNorth position of the panels has a tensile stress at North position whenthe panel is cooled down. The intention of the inventive cooling processis to minimize temperature differences between center and edge of thefront of the panel. By this the tensile stress at the North positionwill decrease, so cracks starting at the North position will more likelypropagate to the corners instead of the center; this is better forsafety. The local reduction in cooling can be advantageously realized byplacing a heat reflection means opposite to the central portion of thepanel which heat reflection means leaves the edge portions free. Whathas been explained with respect to the North position (the central areaof the upper long side of the panel) also holds, mutatis mutandis, withrespect to the East, South, West—and corner-positions.

Reducing the heat radiation of the thinner, central portion, of thepanel has been found to be an effective means to reduce temperaturegradients over the panel surface. Radiation transport is responsible for40-70% of the heat transport. Cooling with air of other portions aloneis less efficient. This effect may for instance be used in practice tomanufacture panels with a lower weight, or panels with a flatter frontsurface, or to reduce the fall-out (=percentage of panels that does notpass safety tests) or any combination of these beneficial effects.

According to a first embodiment before removing the panel from the mold,a heat reflection means is arranged in a position facing the innerportion of the central panel portion.

According to a further embodiment a heat reflection means is used whichcomprises at least one plate made of a material selected from the groupcomprising Ni, Al, Au, or Al-oxide, or a steel plate coated with such amaterial.

To increase the effect of the heat reflection means an edge coolingmeans (fluid cooling) may be arranged adjacent at least one of the edgeportions of the glass panel. (The edge portions are the areas where theviewing window and the peripheral side walls of the glass panel join.)This edge cooling assist the heat reflection means in rendering thetemperature distribution over the panel face more equal.

In view of the above it is in particular an aspect of the invention toprovide a display panel which has a central portion which issubstantially thinner than the edge portions and which presentssubstantially no tensile stresses in the central area of at least one ofthe long and short edge portions of the panel.

A manner to realize this is to take measures that during cooling down inthe forming mold all panel positions pass T_(g) substantially at thesame moment. (T_(g) is the transition temperature at which the glasschanges from the viscous state to the solid state.)

It is known that the contour of the inner face of panels of the typedescribed become distorted during the cooling of the panels immediatelyfollowing the forming thereof. Heretofore, a correction of suchdistortion was usually accomplished by directing a stream of aeriformcooling fluid to a selected area of the panels prior to removal of thepanels from their forming molds, such stream of cooling fluid causingdifferential cooling of the panels which apparently counteracts thecauses of undesirable distortion. There may be situations that theinventive heat radiation control is not compatible with the above methodof correcting contours on the press.

According to a further aspect of the invention directing a stream ofcooling fluid to a selected area of the panel for correcting the innerface contour is carried out after removal from the mold, duringtransport (e.g. on a conveyor belt) to a next processing stage (e.g.annealing in an oven).

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 is a schematic view of a cross-section of a display tube, havinga press-formed glass panel;

FIG. 2 illustrates the method of press-forming;

FIG. 3 illustrates the arrangement of the glass panel during a coolingstep in accordance with the invention.

FIG. 4 shows a glass panel during transport on a conveyor belt.

The Figures are purely schematic and not drawn to scale. In particularfor clarity, some dimensions are exaggerated strongly. In the Figures,like reference numerals refer to like parts, whenever possible.

FIG. 1 is a schematic view of a cross-section of a display tube I havinga glass envelope 2 which includes a display panel 3, a cone 4 and a neck5. In the neck 5, there is an electron gun for generating one or moreelectron beams 9. The electron beam is focused on a phosphor layer 7 onthe inner surface of the display panel 3 and deflected across thedisplay panel 3 in two mutually perpendicular directions by means of adeflection coil system 8.

Display devices often comprise cathode ray tubes or television displaytubes 1 which are entirely made of glass and are built up of two or moreportions with glass walls of different thicknesses or differentheat-absorption characteristics. For example, a glass television displaytube 1 customarily comprises a glass display panel 3 and a glass cone 4which are separately produced and subsequently united by fusing or usinga (solder) glass fit, the joint formed being hermetically tight. Thedisplay panel 3 of such tubes is formed by a glass wall whose thicknessis much greater than the wall thickness of the cone parts of such tubes.Such a greater wall thickness of the display panel 3 serves to ensurethat it is sufficiently rigid when the eventual tubes comprising such apanel are evacuated.

FIGS. 2A and 2B illustrate the method of press-forming. In a firstmethod step (FIG. 2A) a glass volume 21 at a high temperature (typically1100° C.-1000° C.) is supplied to a press 22 having a lower mould member23 a whose form corresponds to the form of the glass panel to be madeand a ring member 24. A glass panel is press-formed in the usual mannerby pressing a plunger 23 b in the mold member 23 a, with the glassvolume 21 in between (FIG. 2A). The warm glass which is in contact withthe relatively cold press will cause the temperature and in particularthe surface temperature of the glass to drop. After formation of theglass panel 3 the plunger 23 b is pulled back and the panel cools downin the mould member 23 a during a certain period of time (e.g. 0,5-5minutes) before the ring member 24 is removed and the panel 3 is takenout (FIG. 2C)

Because of the glass wedge of Real Flat (RF) panels, the center of thesepanel is thinner as the edge of the panel. The thinner part willnormally cool down faster because of the lower heat capacity. Localtemperature differences over the surface of the panel during coolingdown will lead to so-called “membrane” stresses. To eliminate thesestresses it is best to have an equal temperature over the surface.(Note: in the depth, because of the cooling down there is a temperaturegradient, which causes the parabolic stress distribution over thethickness of the panel)

To get an equal temperature over the surface during cooling down, it isnecessary to adapt the cooling to the local thickness of the glass.Cooling with heat flux Q is given byQ=Q _(rad) +Q _(conv)=4εσTm ³(T _(glass) −T _(sur))+h _(conv)(T _(glass)−T _(sur))the convective part=Q _(conv) =h _(conv)(T _(glass) −T _(sur))=v ^(x)(T_(glass) −T _(sur))the radiation part=Q _(rad) =h _(rad)(T _(glass) −T _(sur))=4εσTm ³(T_(glass) −T _(sur))

-   v=velocity of cooling air-   ε=emission coefficient of surroundings-   T_(sur)=temperature of surroundings

So local cooling can be adapted by:

local variation in emission coefficient ε

local variation in velocity of cooling air v

local variation in temperature of surroundings T_(sur) (local heating ofsurroundings)

At 500-600° C., h_(rad)(ε=1)=4*1*56.7E-9*(550+273)³=100[W/m²K], so theradiative heat transfer is comparable to a strong convective heattransfer.

The radiative heat transfer can easily be changed by changing theemission coefficient, where changing the convective heat transportrequires more effort. Therefore the inventions alters the local coolingof a panel by using a radiation controlling means which comprises:

a centrally located plate 27 for reflection, ε_(r)=low, optionally incombination with

an edge cooling means 26, ε_(a=high.)

The central portion of the panel 3 “sees” the reflection plate 27, theedges of the panel 3 “see” the cooling means 26. See FIG. 3. The edgecooling means 26 directs cooling fluid (air) 25 towards the edges of thepanel 3.

A suitable reflection plate should be able to reflect radiation in theinfrared region. E.g. nickel plates, aluminum plates and (polished)Al-oxide plates have been found to be suitable. The dimensions of theplate depend a.o. on the thickness of the edge portions (these may betwo times thicker than the central portion) and the panel-platedistance.

As shown in FIG. 4, cooling fluid 30, 31 for correction of the innerface contour of the panel 3 can be directed onto the outer and/or theinner surface of the panel 3 during transport (in the directionindicated by the arrow 33) on a conveyor belt 34 after removal of thepanel from its forming mold. An (aeriform) cooling fluid can be directedby means of nozzles 29 a, 29 b, or through openings 32 a, 32 b, 32 c, 32d in a cover.

In general terms the invention relates to a method for press-forming acathode ray tube panel, in which, before the panel is removed from thepress, a heat radiation controlling means is provided at a smalldistance from the panel, parallel to the panel surface. Said meanscomprises an area with a high reflectivity for heat radiation, whichfirst area is near the center of the panel. The result of this measureduring the cooling of a panel is a more even temperature distributionover the surface of the panel. In particular, the temperature gradientbetween center and edges of the panel is reduced.

Because of the glass wedge of (Real) Flat panels the center of the panelis thinner than the edge of the panel. The thinner part will normallycool down faster because of the lower heat capacity. Local temperaturedifferences over the surface of the panel during cooling down will leadto so-called ‘membrane’ stresses. The method of the invention willminimize these stresses.

1. A method for manufacturing a glass panel comprising a viewing windowand peripheral side walls for a display tube, comprising a first stageof press-forming molten glass put in a mold using a plunger and a secondstage of cooling the formed glass after it has been taken out from themold, characterized in that during a part of the first stage after pullback of the plunger the heat radiation of the inner face portion of thecentral panel portion is reduced to reduce a temperature gradient to theedge portions of the panel.
 2. A method as claimed in claim 1,characterized in that for reducing the heat radiation of the centralpanel portion before removing the panel from the mold, a heat reflectionmeans is arranged in a position facing the inner face portion of thecentral panel portion.
 3. A method as claimed in claim 2, characterizedin that a heat reflection means is used which comprises at least oneplate made of a material selected from the group comprising Ni, Al, Au,Al-oxide, or a plate coated with such a material. 4 A method as claimedin claim 2, characterized in that simultaneously with the heatreflection means an edge cooling means is arranged adjacent at least oneof the edge portions of the glass panel.
 5. A display panel having acentral portion which is substantially thinner than the edge portionswhich presents substantially no tensile stress in the central area of atleast one of the long and short side portions of the panel.
 6. A methodas claimed in claim 1, characterized in that after removal from the moldthe panel is transported to a next processing stage while a stream ofcooling fluid is directed to a selected area, or selected areas, of thepanel for correcting the contour of the inner face of the panel.